Method and system for performing call admission control in the uplink for third generation wireless communication systems

ABSTRACT

A method and system for performing call admission control in wireless communication systems is disclosed. Resource units required by a new user are assigned based on an outage probability of each uplink timeslot. The outage probability of each timeslot is updated as the resource units are assigned so that each resource unit assignment results in the lowest possible contribution to total outage probability. Once all of the resource units are assigned, the total outage probability is computed based on the resource allocation. If the total outage probability is below a predetermined value, the new user is admitted. If the total outage probability is above the predetermined value, the new user is rejected.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/365,355, filed on Mar. 14, 2002, which isincorporated by reference as if fully set forth.

BACKGROUND

[0002] The present invention relates to the field of communications,specifically wireless communications. More specifically, the presentinvention relates to call admission control in third generation wirelesssystems.

[0003] Third generation wireless communications, such as wideband codedivision multiple access time division duplex (WCDMA-TDD) systems, willsupport not only voice service, but also a wide range of broadbandservices, such as video and Internet traffic. In such a system, the goalof call admission control is to guarantee that the quality of service(QoS) is met for all users admitted into the system. Call admissioncontrol directly affects the QoS of mobile users, and the stability andcapacity of the system. Therefore, call admission control is veryimportant for the design of WCDMA-TDD systems.

[0004] In recent years, there have been some advances regarding calladmission control in WCDMA-FDD systems but few in WCDMA-TDD systems. Onesuch system addresses the problem by making resource allocation based ona fixed required signal to interference ratio (SIR). In WCDMA-TDDsystems, however, the required SIR of a user is not fixed and, incontrast, changes with time because of imperfect power control. InWCDMA-FDD systems, there are no timeslots whereas in WCDMA-TDD systems auser can use more than one timeslot.

[0005] A need therefore exists for providing call admission control forTDD systems.

SUMMARY

[0006] The present invention is a system and method for performing calladmission control where admission decisions are based on a dynamic SIRrequirement and the assumption that a user can use multiple timeslots.The present invention is implemented without using online measurement,thereby avoiding software and hardware implementation costs attributedthereto.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0007]FIG. 1 is a method for performing call admission control in theuplink for third generation wireless communication systems in accordancewith the preferred embodiment of the invention.

[0008]FIG. 2 is a call admission control system in accordance with thepreferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0009] In accordance with the present invention, call admission controlis performed in WCDMA-TDD systems (where users can use multipletimeslots) while taking into account the fact that each user's requiredsignal to interference ratio (SIR) is a random variable. Resourceallocation is optimized so as to yield the lowest total outageprobability (P_(out-total)) for a new user and to ensure P_(out-total)is below a predetermined value.

[0010] The present invention is preferably implemented using thefollowing assumptions. First, as specified by the Third GenerationPartnership Project (3GPP) standards, each frame is divided into 15timeslots. Second, the chip rate of a WCDMA-TDD system is 3.84 Mcpsmaking the equivalent chip rate in one timeslot 256 kcps (i.e. 3.84Mcps/15=256 kcps). Third, a multi-user detection (MUD) receiver is usedat the base station (BS).

[0011] In each timeslot, Orthogonal Variable Spreading Factor (OVSF)codes are used for channelization codes. The spreading factor of achannelization code can take a value of 2, 4, 8, and 16 in the uplink.For purposes of describing the present invention, a resource unit (RU)corresponds to a particular physical channel and is defined as achannelization code having spreading factor 16 in a particular timeslot. RUs therefore correspond to physical channels in a particulartimeslot.

[0012] For a new user seeking admission to a cell, the primary goal ofcall admission control is to properly allocate RUs (i.e. physicalchannels) so that QoS requirements are guaranteed, for both the new userand any users already in the cell. The number of RUs required by a newuser depends on the type of call the new user has placed. For example, anew user placing a voice call requires two RUs while a new user placinga 64 k data call requires five RUs.

[0013] Decisions made by a call admission control system are based onwhether RUs can be allocated successfully for the new user. Whether a RUcan be allocated successfully for a new user depends on the individualoutage probabilities (Pout ) for all of the timeslots in which RUs havebeen assigned. Therefore, P_(out) is the probability that, in aparticular timeslot, a user's required SIR will be below a certainpredetermined value. In WCDMA-TDD systems, however, the required SIR ofeach user is not fixed, but follows a certain distribution therebymaking P_(out) difficult to calculate. That is, even though thedistribution of the SIR is known, the computation of P_(out) is stillvery complex, and cannot be done in real time.

[0014] The Gaussian approximation, in contrast, provides a sufficientlyapproximate result and has relatively low computation complexity.Therefore, the Gaussian approximation approach is used to allow the RNC(Radio Network Controller) to compute P_(out) for each timeslot and makeresource allocation decisions in real time.

[0015] The P_(out) of every timeslot assigned to a new user may becombined to compute P_(out-total) for the new user. Assuming a new useris allocated RUs in a particular number of timeslots, the P_(out-total)of a new user is defined as the probability that an outage will occur inat least one of those timeslots. The P_(out-total) may be computed asdesired. By way of example, P_(out-total) may be computed according to${P_{{out} - {total}} = {1 - {\prod\limits_{i = \Omega}\left( {1 - {P_{out}(i)}} \right)}}},$

[0016] where Ω is the set of timeslots in which RUs have been allocatedto the user.

[0017] Referring now to FIG. 1, a method 10 is shown wherein calladmission control is performed in the uplink for third generationwireless communication systems. Assuming, purely for purposes ofdescribing the invention, that a new user requires two RUs (i.e. the newuser has placed a voice call), the method 10 begins in step 12 bycomputing the current P_(out) of each uplink timeslot. Again, P_(out) isthe probability that a new user's SIR is below a predetermined value ina particular timeslot and is computed for each uplink timeslot.Therefore, in step 12, the probability of the new user's SIR being belowthe predetermined value is computed for each timeslot. As explained,P_(out) accounts for the fact that the user's SIR changes with time andis computed by the RNC using the Gaussian approximation to reducecomputation complexity.

[0018] Once P_(out) has been computed for each timeslot, the timeslothaving the lowest P_(out), say timeslot i, is selected in step 14. Sincetimeslot i is the timeslot with the lowest P_(out), the P_(out) intimeslot i is denoted P_(out)(i). In step 16, one RU is assigned totimeslot i and P_(out)(i) is updated accordingly. Once the first RU hasbeen assigned, the method proceeds to step 18. In step 18, the methoddetermines whether additional RUs need to be assigned. As mentioned, forpurposes of describing the invention, it can be assumed that the newuser requires two RUs. Therefore, the determination in step 18 will bepositive and the method will proceed to step 20.

[0019] In step 20, the method determines whether P_(out)(i) is still thelowest P_(out) (i.e. the method determines whether, despite beingassigned a RU, timeslot i still has the lowest P_(out)). If P_(out)(i)is still the lowest P_(out), the method goes back to step 16 and thesecond RU is assigned to timeslot i and continues as indicated. If, incontrast, P_(out)(i) is no longer the lowest P_(out), the methodproceeds to step 22. In step 22, P′_(contribution) is computed. TheP′_(contribution), is the contribution to P_(out-total) assuming thenext RU (i.e. the second RU according to the assumption noted above) isaccepted to timeslot i despite the fact that P_(out)(i) is no longer thelowest P_(out). The P′_(contribution) is the same value as the newP_(out) of timeslot i. That is, P′_(contribution) is equal toP_(out)(i)′.

[0020] In step 24, P_(contribution) is computed. The P_(contribution) isthe contribution to P_(out-total) assuming the next RU (i.e. the secondRU according to the assumption noted above) is accepted to the timeslothaving the lowest P_(out), say timeslot j. The P_(contribution) is givenby P_(contribution)=1−(1−P_(out)(i)). (1−P_(out)(j)). OnceP′_(contribution) and P_(contribution) have been computed, the methodproceeds to step 26 where it determines whether P_(contribution) isgreater than or equal to P′_(contribution) (i.e. P_(out)(i)′). IfP_(contribution) is greater than or equal to P′_(contribution), themethod proceeds to step 16 wherein the next RU will be assigned totimeslot i despite the fact that timeslot i no longer has the lowestP_(out). That is, even though timeslot i no longer has the lowestP_(out), assigning the next RU to timeslot i will result in a lowerP_(out-total) than assigning the next RU to timeslot j, which actuallyhas the lowest P_(out). If, in contrast, P_(contribution) is less thanP′_(contribution), i is set equal to j in step 28 and the methodproceeds to step 16. The method sets i equal to j so that, in step 16,the next RU is assigned to timeslot j because assigning the next RU totimeslot j will result in the lowest P_(out-total).

[0021] From step 16, the method again proceeds to step 18. Note, steps20 through 28 would not have been necessary where the new user onlyneeded one RU. But, because in the assumption of the example the userneeded two RUs, one run through steps 20 through 28 was necessary inorder to determine the optimal allocation of the second RU. Steps 20through 28 are performed, as needed, for every RU required by the user.Once all of the RUs have been assigned, the method proceeds to step 30.In step 30, P_(out-total) is computed to determine the outageprobability of the new user based on the allocation of RU(s), asallocated in steps 12 through 28.

[0022] In step 32, the method determines whether P_(out-total) is lessthan or equal to a predetermined value, say θ. The predetermined value θis an operator dependent parameter and may be any value, as desired,depending on the desired level of network stability. If P_(out-total) isless than θ, the new user is admitted (step 34). If not, the new user isrejected (step 36).

[0023] Pursuant to the present invention, P_(out-total) increases as thenumber of users increases and saturates around the predetermined value θthereby dramatically improving system stability (i.e. the number ofdropped calls). Due to the stringent admission standards, the presentinvention also results in a dramatic increase in blocking probability(which also increases as the number of users) in comparison to staticsequential and random call admission control methods. The combination ofincreased stability and blocking probability significantly improvesusers QoS as, from a user's perspective, having a call blocked is muchmore preferable than having a call dropped.

[0024] Referring now to FIG. 2, a system 100 is shown for implementingcall admission control according to the present invention. The system100 comprises a RNC 102, a BS or Node-B 104 and user equipment (UE) 106wherein the BS and UE each have a multi-user detection (MUD) receiver103, 108, respectively.

[0025] When the UE 106 is used by a user to place a call, the RNC 102will perform call admission control and allocate RUs required by thatnew call to appropriate timeslots so as to ensure the lowest possibleP_(out-total) and to ensure that P_(out-total) remains below thepredetermined threshold θ.

[0026] To perform call admission control, the RNC 102 computes P_(out)for every uplink timeslot and assigns a RU to the timeslot with thelowest P_(out). If there are additional RUs required by the new userthat need to be allocated, the RNC 102 will assign subsequent RUs to thesame timeslot the previous RU was assigned to, so long as that timeslotstill has the lowest P_(out). If that timeslot no longer has the lowestP_(out), the RNC 102 will determine whether it still should assignsubsequent RUs to that timeslot or to the timeslot now having the lowestP_(out). To make that determination the RNC 103 determines whichtimeslot results in the lowest contribution to P_(out-total). The RNCrepeats this analysis for every RU required by the new call.

[0027] Once all of the RUs that are required by the new user have beenallocated to particular timeslots, the RNC 103 determines whether theallocation results in P_(out-total) being below the predetermined θ. IfP_(out-total) is below θ, the new user is admitted. If not, the new useris rejected.

[0028] Although the present invention has been described in detail, itis to be understood that the invention is not limited thereto, and thatvarious changes can be made therein without departing from the spiritand scope of the invention, which is defined by the attached claims.

What is claimed is:
 1. A method for performing call admission control comprising: assigning resource units required by a new user to timeslots based on outage probability of each individual timeslot; computing a total outage probability for the new user; determining whether the total outage probability is below a predetermined value; and admitting the new user where the determination is positive.
 2. A method as in claim 1 wherein the step of assigning resource units is performed by: computing an outage probability for each timeslot; assigning a first resource unit to the timeslot having the lowest outage probability; and assigning subsequent resource units to timeslots that will result in the lowest total outage probability for the new user.
 3. A method as in claim 2 wherein the timeslots that will result in the lowest total outage probability are selected based on each of their individual contributions to total outage probability.
 4. A method for performing call admission control comprising: computing an outage probability for each uplink timeslot; assigning a first resource unit required by a new user to the timeslot having the lowest outage probability; assigning subsequent resource units required by the new user to timeslots that will result in the lowest total outage probability for the new user; computing a total outage probability based on which timeslots have been assigned resource units; and admitting the new user if the total outage probability is below a predetermined value.
 5. A method as in claim 4 wherein the timeslots that will result in the lowest total outage probability for the new user are identified by identifying the timeslots that contribute the least to the total outage probability of the user.
 6. A method for performing call admission control comprising: computing an outage probability for each timeslot; identifying a timeslot having the lowest outage probability; assigning a first resource unit required by a new user to the identified timeslot; assigning subsequent resource units required by the new user to a group of timeslots that minimizes total outage probability of the new user; computing a total outage probability based on which timeslots have been assigned resource units; and admitting the new user if total outage probability is less than a predetermined value.
 7. A method as in claim 6 wherein each timeslot in the group of timeslots is selected based on its individual contribution to total outage probability.
 8. A method as in claim 6 wherein the step of assigning subsequent resources further comprises: computing a first value indicative of the contribution to the total outage probability if a subsequent resource unit is assigned to the identified timeslot; computing a second value indicative of the contribution to the total outage probability if a subsequent resource unit is assigned to a timeslot other than the identified timeslot; continuing to assign subsequent resource units to the identified timeslot as long as the second value is not greater than or equal to the first value; and assigning subsequent resource units to a timeslot other than the identified timeslot if the second value is greater than or equal to the first value.
 9. A call admission control system comprising: user equipment; a base station; and a radio network controller wherein the radio network controller is adapted to: assign resource units required by a new user to a plurality of timeslots based on each timeslots individual contribution to total outage probability of the new user; and admit the new user where the total outage probability of the new user is below a predetermined value.
 10. A call admission control system as in claim 9 wherein the radio network controller is adapted to: compute an outage probability for each uplink timeslot each time a new user seeks admission; assign a first resource unit required by the new user to the timeslot having the lowest outage probability; assign subsequent resource units required by the new user to a group of timeslots that will result in the lowest total outage probability for the new user; and admit the new user if total outage probability is below a predetermined value.
 11. A call admission control system as in claim 10 wherein each timeslot in the group of timeslots is selected based on its individual contribution to total outage probability.
 12. A call admission control system as in claim 10 wherein the radio network controller is configured to select the group of timeslots by: computing a first value indicative of the contribution to the total outage probability if a subsequent resource unit is assigned to the identified timeslot; computing a second value indicative of the contribution to the total outage probability if a subsequent resource unit is assigned to a timeslot other than the identified timeslot; continuing to assign subsequent resource units to the identified timeslot as long as the second value is not greater than or equal to the first value; and assigning subsequent resource units to a timeslot other than the identified timeslot if the second value is greater than or equal to the first value. 